In recent years, various methods, in which encoded data realized by high-efficiency compression based on interframe prediction are transmitted, have been used as methods for effectively transmitting moving picture image data. In these transmission methods, a reduction in the amount of information of moving picture image data that have a high correlation in the time direction can be achieved by encoding predicted parameters and predicted residual image data that are obtained by predicting encoded images based on frames that precede and follow in time. In addition, conversion encoding and quantizing the predicted residual moving picture data achieves compression encoding with greater efficiency and thus enables transmission of moving picture image data with a limited transmission bandwidth.
Representative examples of the above-described transmission method include methods that use compression encoding methods such as MPEG (Moving Picture Experts Group)-1, MPEG-2, and MPEG-4. In these compression encoding methods, input image frames are subjected to interframe prediction by motion compensation in units of rectangular regions having a fixed size referred to as “macro-blocks,” and the obtained motion vectors and signal data that have been compressed by subjecting the predicted residual image data to two-dimensional discrete cosine transform and quantization are subjected to variable length encoding.
Many methods exist for distributing compression encoded moving picture image compression information such as described above by way of an IP (Internet Protocol) network that uses a packet exchange method. It is expected that in the future, the distribution of moving picture images will be evolved in a mobile communication system in which mobile stations that are constituted by PHS (Personal Handyphone System), portable telephones, or portable terminals that use these devices as communication means are connected by way of radio channels to a radio base station.
However, in a mobile communication system, handover occurs when a mobile station exceeds a radio area (cell) and moves into a neighboring radio area. Normally, the reception level of the radio channel by which communication is realized is monitored in the mobile station, and when handover causes the reception level to fall below a prescribed level, a handover request is transmitted to the network side, and on the network side, the connection destination of the mobile station is switched from the base station before movement to a base station following movement in accordance with the handover request from the mobile station.
However, during a handover interval, the reception of data is halted, bringing about the loss of a large amount of data and deterioration of the quality of the received image. In an encoding method that uses interframe prediction such as MPEG in particular, data that are not subject to prediction are received after the completion of handover, and the quality of the moving picture image data during reception therefore undergoes serious deterioration. This image disturbance also influences succeeding frames.
Mobile communication systems that can reduce the loss of data resulting from handover have been proposed (refer to JP-A-2000-152307 (hereinbelow referred to as the “Patent Document 1”)). This mobile communication system is provided with a plurality of base stations that are connected to a network and mobile stations that are selectively connected to these base stations by way of radio channels. A mobile station transmits a request to execute a broadcast procedure to the network when the reception quality of the radio signal from the currently connected base station falls below a prescribed level due to handover. On the network side, communication information addressed to the mobile station is broadcast to the base station that is currently connected to the mobile station and other base stations that are in the vicinity of the currently connected base station in accordance with the execution request from the mobile station. Communication information is then broadcast to the mobile station from the currently connected base station and other base stations that are in the vicinity. By means of this broadcast transmission, the mobile station can instantly obtain the communication information that is addressed to itself upon the completion of handover, and as a result, can reduce the loss of data that is caused by handover.